Patent GB-1248050-A (GB)
Title: Step and repeat cameras
Inventor: ABLES BILLY DEAN
IPC: G03B27/44
Filed: 1968-10-18
Abstract: 1,248,050. Step-and-repeat cameras; Interferometers; Mounts for optical apparatus. TEXAS INSTRUMENTS Inc. Oct.18, 1968 [Oct.18, 1967(6)], No.49461/68. Headings G2A and G2J. In a step-and-repeat camera for producing photo-masks for fabricating large arrays of integrated circuits, an image is projected on to a photographic plate supported on a table, the table having drive means for moving it in X and Y directions, interferometer means for sensing the said movements, and computer means for controlling the movement of the table by continuous reference to the interferometer fringe counts. As shown in Fig.1, the camera is mounted on a base casting 14, itself resting on a massive concrete block 12. A granite block 18 having a very accurately planar upper surface 22 is supported on casting 14 by three levelling screws 20. The film support table 26 comprises a granite block 42 supported by four constant pressure air bearings 46, see also Fig.3, which ride on the surface 22, thus providing for low-friction movement of the table. Fig.3 shows a precision guide and drive system for the table. This comprises a pair of glass bars 48, 50 mounted on the granite block 18. On either side of these are two granite slabs 52, 54 connected by a third slab 56. On slabs 52, 54 are two further glass bars 70, 72. The side edges of the glass bars are optically flat, and are bridged by U-shaped yokes 58, 60 fixed to slab 56 and by yokes 74, 76 fixed to block 42. The yokes ride along the glass bars, being spaced from the optically flat edges by air bearings. Thus the granite block 42, and hence table 26, is moved in the X-direction along the guide bars 48, 50 by a motor 86 frictionally driving a bar 90 attached by a rod 92 to yoke 58, and is moved in the Y-direction by a motor 98 frictionally driving a bar 102 attached by a rod 104 to yoke 76. The granite block 42 forming the lower part of table 26 supports a metal casting 44 forming the upper part. This receives a pair of multiple plate carriers 120, shown in more detail in Figs.5 and 9. Each plate carrier comprises a base 122 and a lid 126, connected by hinges 128, 130. The base is divided into nine identical compartments separated by interior walls 132, and aligned square openings are provided in the base (e.g. 122a) and in the lid (e.g. 126a). Each compartment is adapted to receive a square glass photographic plate 134, means being provided to hold the plate in a precisely oriented position relative to the carrier. A similar pair of carriers 120 carrying the master transparencies are mounted in the upper stage 32 of the camera which also carries a pair of lamp houses 36, 38. The lower camera stage 34 carries the lenses, and both stages are mounted for vertical adjustment on uprights 28, 30 secured to the base casting 14. Machined reference surfaces 170, 172, 174 on the carriers enable them to be located with precision in the casting 44. The two stages 32, 34 can be adjusted vertically on the uprights 28, 30 with great accuracy by means of optical devices 380, 382, and 484, 486 which read against 310, 328. The stages move under the action of electric motors, and each end of the stage can be set separately, and then locked in position by pneumatic brakes. Each of the lamp houses 36, 38 on the upper stage contains nine lamps, and the lower stage carries a corresponding number of lens barrels, Figs.26, 27 (not shown) each holding a similar reducing lens. Each lamp is connected to its corresponding lens by a bellows 40, so that there are eighteen separate systems. Six of the bellows can be seen in Fig.1. The position of the table 26 in the X and Y directions is measured by detecting and counting the inteference fringes of a laser interferometer resulting from the movement of the table from a reference position. The interferometer comprises a helium-neon laser 516 Fig. 28 which projects a beam of light on to a partially silvered mirror 518. The transmitted part 519 of the beam continues to a second such mirror 520. The reflected part of this beam falls on an adjustable mirror 524 on table 26. Some of this light is reflected back to an optical system 540. The part of beam 519 passing through mirror 520 falls on a fixed mirror 527, and some of the light reflected from mirror 527 is also reflected from mirror 520 to the device 540. Mirror 527 is precisely perpendicular to the beam path, but mirror 524 is adjusted to be slightly out of perpendicular. Thus the two coherent beams reaching the device 540 combine to produce interference fringe lines, which pass through a complete sinusoidal cycle of illumination each time the table moves a half-wavelength in either direction along the X-axis. A similar arrangement is provided for light reaching a second optical device 550, for table movement along the Y-axis. Device 540 comprises a pair of lenses 542, 544 and a pair of mirrors 546, which divide the beam into two spaced beams received by respective photo-diodes 548, the output being passed to an amplifier and X-fringe detector. Similarly the beam reaching device 550 is received by a pair of photo-diodes 552, the output being passed to an identical amplifier and Y-fringe detector. The movement of the table is controlled by a computer which detects the fringe counts, and the table is initially set at the origin X o , Y o . The computer then operates the drive system in such a manner as to move the table to the exposure position by continuously computing the position and velocity of the table from the readings of the fringe counters. Then the table is maintained at the exposure position during the exposure period by continuously determining the position of the table from the fringe counters and operating the drive system to produce forces for correcting the positional error. Since the wavelength of light in air varies with temperature, pressure, and humidity, the camera is located in a room in which the temperature and humidity are maintained substantially constant. The barometric pressure is measured on a gauge, and the information fed to the computer, which makes a continuous correction.